Generally, an electronic document such as a Hancom Hangul file, an MS Word file, an Adobe PDF file, etc. is a document written with a computer, etc. and composed of digital information. Unlike conventional documents composed of paper, the electronic document can be quickly transmitted through a data communication network, the internet network, etc. Thus, the use of the electronic document is rapidly increasing due to development in computer-based communication technology.
However, the electronic document is transmitted as digital signals through a communications network and thus security of the document is poor. Therefore, as one method of enhancing reliability of the electronic document, a method of attaching a digital signature is used.
A digital signature is a seal, a signature, or a cipher-form signature composed of digital information verifying a person who sends an electronic document or digital data (or a message). Also, the digital signature may be used to guarantee that original content of the transmitted electronic document or digital data is not falsified. Therefore, a user attaches the digital signature composed of digital information to the electronic document or the digital data instead of putting a seal or a signature on a document.
That is, in online contracts, electronic transactions requiring an electronic seal, etc., a digital signature may be used to certify a relevant electronic document or digital data. Recently, the digital signature has been actively used as a means of assuring reliability of electronic transactions with legal effect.
Also, a digital signature is performed by a signer in an electronic document or in a service such as electronic contracts, or electronic offers, etc. on the web. In a digital signature, a digital certificate may be used to certify the digital signature. The digital certificate is a certificate issued by national authorized institutions, and identifies a signer or identifies that the digital signature of the signer is authentic.
An additional benefit that can be obtained by using the digital signature as described above is that the digital signature can be easily transmitted; the digital signature cannot be easily denied; the digital signature cannot be forged; and a timestamp may be automatically maintained. Regardless of whether or not the digital signature is encrypted, the digital signature may be used in any kinds of electronic documents or digital data. Therefore, a receiver may trust that the electronic document or the digital data is intactly transmitted without being falsified and identity of a sender.
Recently, as computer-based crime technology has become more sophisticated, it is impossible to identify falsification or forgery of an electronic document and/or digital signature through a communications network in transmission process of the electronic document. Thus, the digital signature is not able to fulfill its original purpose.
In the meantime, in conventional verification of forgery or falsification of an electronic document, only file integrity can be identified by comparing hash values of an electronic document file. When the electronic document is forged or falsified, it is required to visually compare an original electronic document file with a verification target electronic document or with a forged or falsified electronic document file by being presented to a viewer or being printed in order to identify the location of forged or falsified content of the electronic document.
Moreover, forgery or falsification of the electronic document file means that the content of the electronic document is different from the original. Accordingly, it is extremely important to identify what is different from the original, and the location of the changed content. Thus, it is necessary to indicate and identify the forgery falsification location with integrity verification of the electronic document file.